Alright. The brain takes in external stimuli through the senses, and forms a picture of the external world on the basis of this stimuli. Fire burns. Water is wet. Ice is cold. The sun is bright. As rational creatures we are capable of conscious thought, including the ability to form a theory of mind--I have a mind, and other minds exist also independent of my own. We communicate what happens in our brains through language, in particular the spoken/written word by which we can communicate very complex ideas from which, ultimately, civilization can happen. We have developed a method by which to measure observable phenomenon and make sense of the external world, that's science. And science does an expert job in giving us understanding about how the external world operates, even in cases where our own immediate perceptions don't readily help us--often through another important discipline, mathematics.
Part of that language, civilization, society thing is that as human beings dwelling in complex social groups we share our ideas through language. From this springs poetry, art, philosophy, music, and religion. In many cases human beings need to make a decision to believe a thing or not believe it, in some cases this is made easy by direct experiential knowledge (e.g. yes, fire burns, I believe it because I've been burned by fire or I saw my friend get burned by fire) or through (again) the scientific method which does an expert job at making sense of observable reality through its own methodology. But not all ideas are as simple as this, in many cases ideas may really be entirely subjective, such as whether or not a particular piece of music is good or bad. In other cases it's because we are discussing ideas that may or may not correspond to objective reality, but with which we have no immediate access because it lay outside of our senses. For example, do gods exist? We have no testable means to discover whether or not this is true, all we have are ideas and, occasionally, the claims of unique experiences which are unable to be ascertained as true or not. In which case we then formulate a belief on whether or not we regard it to be trustworthy, and the processes by which we reach a conclusion on whether a belief is trustworthy can be as unique to an individual as the individual themselves.
As a Christian I am part of a religious tradition and community that goes back two thousand years to when a relatively small group of Jewish people became convinced that the historical figure known as Jesus of Nazareth was, in fact, the long-awaited Messiah sent by the God who made Himself known to Abraham, Jacob, and Moses; and that this Jesus though having suffered and died by the hands of the Romans, being crucified, did not remain dead but rose from the dead, ascended to God, and will return at some time as judge of the living and the dead. Thus arose from within the complex socio-political climate of 1st century Judea a Messianic community centered around Jesus of Nazareth, with a set of theological ideas, based upon the claims of some in this early community to having known Jesus and witnessed, first hand, His own resurrection. I receive these beliefs as true as a matter of faith. Others, very clearly, do not hold to these same beliefs, they may hold to other beliefs entirely, or they may subscribe to no sort of religious dogma whatsoever.
How can I hold to these things as true? Faith. Faith that these beliefs correspond to objective reality, even if there is no way to demonstrate that to be the case. I remain, nonetheless, a meme-carrier of this particular religious tradition and community because I maintain, by faith, that what this religious tradition and community has continued to belief down through the centuries does, in fact, correlate to reality.
-CryptoLutheran